Technical Field
The present disclosure relates to a transformer. More particularly, the present disclosure relates to a transformer with shielding winding.
Description of Related Art
A switching power supply conducts switching operations by a switch of the power converter to control the transmission of power. However, the switching operation of the switch may generate the electromagnetic noise, and that is, the operating power converter becomes a noise source for an electrical grid and surrounding equipments. To prevent the severe interference of the noise source, global governments and the related international organizations collectively constitute EMC (electromagnetic compatibility) specification.
The electromagnetic noise includes common-mode and differential-mode noise, and there are two methods resolving the common-mode interference: attenuating the noise source and disconnecting the noise propagation path. Concerning the transformer in the power converter, a primary winding and a secondary winding form the coupling capacitance. Generally speaking, the switching power supply generates propagating interference of the common-mode noise through the coupling capacitance of the transformer.
When two opposite-phase noise co-exist in the circuit of the switching power supply, the common-mode noise of the primary winding circuit and secondary winding circuit can be mutually cancelled out by way of changing magnitude of the coupling capacitance and weakening the overall common-mode noise.
Inserting shielding layers between the primary winding and the secondary winding or adding additional compensation capacitors may change the magnitude of the coupling capacitance. However, the compensation capacitors bring additional cost, and it is not easy to balance the common-mode noise of the primary winding circuit and the secondary winding circuit. Therefore, it is more common to insert the shielding layers between the primary winding and the secondary winding.
Nonetheless, inserting the shielding layers between the primary winding and the secondary winding of the transformer increases the distance between the primary winding and the secondary winding, which magnifies the leakage inductance of the transformer. Furthermore, additional shielding layers increase size and cost of the transformer.
Accordingly, the needs of the unresolved exist in the art to address the aforementioned deficiencies and inadequacies.